Heat-sensitive recording materials are well-known, which utilize the color-forming reaction between a leuco dye and a developer to produce recorded images by heating. Such heat-sensitive recording materials are relatively inexpensive, and the recording apparatuses therefor are compact and easily maintained. Heat-sensitive recording materials have, therefore, found a wide range of uses: they are used not only as recording media for the output of facsimiles and a variety of computers, printers of scientific measuring equipment, etc., but also as recording media for a variety of printers of POS labels, ATMs, CAD, handy terminals, paper for various tickets, etc.
Heat-sensitive recording materials develop color when a leuco dye and a developer melt by heat and come into contact with each other; therefore, sticking is likely to occur, i.e., a phenomenon in which components of the heat-sensitive recording material that has been melted by heat adhere to the thermal head, and the adhered portion is removed as a result of forcible conveyance by a feed roll.
There is a well-known method for solving such sticking problems, which comprises adding to the heat-sensitive recording layer calcium carbonate, clay, talc, urea-formalin resin, amorphous silica or like oil-absorbing filler (see Nonpatent Document 1).
Among such fillers, amorphous silica is especially preferable, because it has high oil absorption, and imparts to heat-sensitive recording materials high degrees of brightness. Examples of proposed recording layers comprising amorphous silica are as follows: a recording layer comprising amorphous silica having a considerably large primary particle diameter, which is 30 nm or more, while having a remarkably small secondary particle diameter, which is 200 to 1,000 nm (see Patent Document 1); a recording layer comprising powdered silicic acid (see Patent Document 2); a recording layer comprising fine particles of silica whose surfaces have been made spherical (see Patent Document 3); a recording layer comprising amorphous silica having a specific oil absorption (see Patent Document 4); a recording layer comprising amorphous silica having an average secondary particle diameter of 3 to 10 μm and a specified oil absorption (see Patent Document 5); and a recording layer comprising amorphous silica (see Patent Document 6). However, further improvements in recording layers are demanded in terms of recording density and the suppression of undesired color development due to sticking or scratching.
Moreover, heat-sensitive recording materials have been proposed which include a heat-sensitive recording layer containing colloidal particles of amorphous silica referred to as “colloidal silica” (substantially composed of primary particles and substantially free from secondary particles that are agglomerates of the primary particles) (see Patent Document 7 and 8). However, further improvements are demanded in terms of recording density and the suppression of sticking.
Nonpatent Document 1: Takashi Shiga, “Kami Parupu Technique Times” (Paper Pulp Technique Times), 27 (8), 34 (1984)
Patent Document 1: Japanese Unexamined Patent Application Publication No. 1984-22794
Patent Document 2: Japanese Unexamined Patent Application Publication No. 1984-26292
Patent Document 3: Japanese Unexamined Patent Application Publication No. 1987-176878
Patent Document 4: Japanese Unexamined Patent Application Publication No. 1995-76172
Patent Document 5: Japanese Unexamined Patent Application Publication No. 1996-310132
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2003-11519
Patent Document 7: Japanese Unexamined Patent Application Publication No. 1993-294065
Patent Document 8: Japanese Unexamined Patent Application Publication No. 2004-25775